drm/amd/display: Remove the repeated declaration

Function 'dc_power_down_on_boot' is declared twice, remove the
repeated declaration.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>